This invention relates generally to a light source for photographic printers and more particularly to an additive light source for color printers.
Various light sources have been used in printing apparatus for providing printing light of proper color composition. For example, a strong white light source may be employed together with successive color filters which pass light in a selected color band. The filtered light combines to provide the proper color composition. Other systems employ lamps providing the three primary colors. The light is filtered as it is directed into a mixing chamber. The intensity of the lamps is controlled to provide light of proper composition.
Mixing chambers have taken various forms. Mixing chambers having diffuse surfaces have been used as well as mixing chambers having highly reflective surfaces. It is also known to provide mixing chambers of various sizes to accommodate negatives of different sizes.
In general, a single lamp for each of the primary colors is employed together with filters. The red sensitive layers in the photographic emulsion of the printing papers are less sensitive than are the green or blue layers. As a result, exposure time is necessarily increased in order to properly expose the red colors. The result is that there is a need for slope control circuits to prevent color shift between prints made with different exposure times and having different color compositions.